You could sidewall vent water heater only if:
it is a direct vent type water heater – it takes all combustion air from exterior. It might be double pipe design (two pipes visible inside) changing to one pipe at wall penetration, or pipe within the pipe inside with a special termination connector on exterior
many tankless water heaters can be vented through the side wall
power vent type water heater can be also vented through the side wall (PVC vent)
Give me the model # and brand of your appliance if you’re not sure, or email me a picture of it.